<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<link rel="stylesheet" href="common.css">
	<style>
		.one{
			position: relative;
			top: 50px;
			left: 50px;
			/*margin-top: 50px;*/
			/*z-index: -1;*/
		}

		.assist{
			position: absolute;
			left: 0;
			top: 0;
			width: 200px;
			height: 200px;
			border:2px dashed black;
		}
	</style>
</head>
<body>
	<div class="box1 one"></div>
	<div class="box2"></div>

	<div class="assist">辅助观察线</div>
</body>
</html>